1. Field of the Invention
The present invention generally relates to the interconnection of electronic components to carriers and, more particularly, to an improved process for applying bonding agents, such as solder pastes and conductive adhesives, to direct chip attachment (DCA) connection sites on carriers in the manufacture of electronic circuits.
2. Description of the Prior Art
Solder alloys are used pervasively to interconnect components to carriers, such as integrated circuit (IC) chips to modules or printed circuit boards (PCBs). There are many ways to apply a defined volume of solder to each pad on a carrier for attachment of components. For example, the decal process has solder bumps plated to a stainless steel carrier that is aligned and placed on an individual site, then the solder volume is transferred to the pads during an infrared (IR) oven reflow.
The solder inject process aligns and places the inject head on the site. Molten solder is forced out of extremely small nozzles, under pressure, until the solder contacts the pads. The pressure is released very gradually until the solder separates by scapular action, and leaves a defined volume of solder on the pad.
The solder inject molding process uses molten solder under pressure to force solder into a mold cavity either on the site or into a solder holder. The holder is indexed into position on a site, then a heated plate contacts the holder and the solder, transferring the solder to the pads.
While solder interconnection is a proven technology in the fabrication of circuits, there are undesirable attributes of solder alloys including high temperature joining, cleaning of residues, fatigue life and the environmental ramification of lead (Pb), which make solder interconnection unsuitable for some applications. An alternative to solder is electrically conductive adhesives. Lower temperature joining is possible, post cleaning of flux residue is eliminated, fatigue life enhancement is possible, and lead is eliminated. Screen printing 0.004" features on 0.008" centers is possible with some electrically conductive adhesives. However, because of adhesive slumping, more than one screening application is necessary to build up a 0.005" thickness of adhesive. Typically 0.002 to 0.003" thick adhesive is screened. The solvent is dried out with time at a predetermined temperature. A second screening deposits another 0.002 to 0.003" thickness of adhesive. The drying cycle is repeated. Slumping is thus minimized as is shorting between bonding pads. Disadvantages to multiple screenings are (1) the probability for particulate contamination at the interface between successively screened layers and (2) the time associated with two passes through screening and drying.